Role of adhesion in the colonization of barley leaves by the yeast Rhodosporidium toruloides

To investigate whether leaf-surface yeasts adhere to the phylloplane in a two-phase manner, with weak, nonspecific attachment followed by stronger, time-dependent adhesion, we observed adhesion kinetics of the basidiomycete yeast Rhodosporidium toruloides to barley. While 50-60% of the cells adhered in short-term assays (up to 3 h), fewer than 10% were adherent after 10 days. Ten attachment-minus (Atr) mutants, deficient in phase I attachment, did not adhere after 5-day incubations, further suggesting a lack of stronger, independent phase II adhesion. Long-term (5 day) adhesion was similar for two isolates of R. toruloides and the ubiquitous leaf-surface fungus Aureobasidium pullulans. Long-term adhesion of R. toruloides to leaves of a waxless barley mutant was significantly greater than to leaves of the wild-type cv. Bonus. Application of exogenous nutrients (dilute yeast carbon base) to resident, wild-type R. toruloides populations resulted in both a rapid recolonization to the apparent carrying capacity of the leaves and an increase in the total adherent populations. Atr mutants recolonized barley segments, when supplied with nutrients, after more than 99% of the cells had been removed by agitation. Therefore, adhesion of R. toruloides to leaves was not required for subsequent colonization of the phylloplane. Overall, these data suggest that the frequency of yeast emigration from leaf surfaces, microbial growth rates, and leaf surface characteristics are major factors influencing colonization of leaf surfaces.